Grease composition, machine component, and starter overrunning clutch

ABSTRACT

A silicone grease composition having high friction characteristics and wear characteristics is disclosed. The silicone grease composition is provided by adding a zinc salt (B) containing, at a mass ratio from 1:99 to 99:1, a zinc dialkyldithiophosphate (B1) and a zinc dialkyldithiocarbamate (B2) as an extreme-pressure additive to a silicone oil (A).

TECHNICAL FIELD

The present invention relates to a grease composition containing asilicone oil. More particularly, the present invention relates to agrease composition which is suited for use in a lubrication part of astarter overrunning clutch, the grease composition containing zincdialkyldithiophosphate and zinc dialkyldithiocarbamate asextreme-pressure additives and containing a silicone oil as a base oil,and the grease composition having a high coefficient of friction andexcellent wear resistance. In addition, the present invention relates toa machine component in which the grease composition is enclosed, andparticularly to an overrunning clutch.

BACKGROUND ART

The difference in thickness—that is, the weight—of a material of amechanical part is determined by the degree to which the safetycoefficient of strength is set at the time of design. In particular, inthe case of a part for an automobile, taking into consideration theeffects on the global environment such as exhaust gas regulation orglobal warming, it is essential to reduce weight by using lightweightmaterials, reducing the thickness, reducing the shaft diameter, or thelike, which will leave little margin in terms of strength. Therefore, toalleviate physical impact with a slip mechanism, a clutch or torquelimiter mechanism is applied to a greater variety of parts than that hadbeen done previously. In addition, an idling STOP for stopping theengine while stopped at a traffic signal has been put into practicalapplication as a countermeasure against automobile exhaust gas, which isone cause of global warming. Therefore, the engine is started with astarter each time the vehicle is restarted after stopping at a trafficsignal, and the frequency with which the engine starts has increaseddramatically.

The part where the torque is the greatest and the conditions are mostsevere in a clutch or a torque limiter mechanism is the overrunningclutch of the engine starter of an automobile. The starter of anautomobile is used in a severe environment in which it is exposed totemperatures ranging from an extremely low temperature of −40° C. to ahigh temperature of 120° C. because it is installed near the exhaustpipe. A grease composition is used for the lubrication of a starteroverrunning clutch. Presently, a grease composition containing asilicone oil as a base oil is primarily used. This is because thecoefficient of friction of a silicone oil is high, but since the surfacetension of a silicone oil is smaller than other oils at 20 to 25dyn/cm², there is a characteristic that a lubricating film is difficultto form and a boundary lubrication state is likely to occur. Thedurability count in the market of overrunning clutches for enginestarters using commercially available silicone grease is currentlyapproximately 30,000 to 50,000 times. When idling STOP is incorporated,the clutch wears down and stops transmitting torque in a short amount oftime (a few years) due to the high frequency of use. As a result, themotor idles, resulting in a state in which the engine does not start.

In this way, the grease composition used in the lubricating part of aclutch or a torque limiter mechanism absolutely needs to have a highcoefficient of friction and excellent enhancement of wear resistance ascharacteristics.

Taking into consideration the circumstances described above,extreme-pressure additives are used as additives for reducing thefriction and wear between two metal surfaces and for preventingsticking, and there are many known grease compositions containingextreme-pressure additives. For example, JP 2010-112235 A describes alubricating oil in which at least one of zinc dialkyldithiophosphate orzinc dialkyldithiocarbamate is added to a silicone oil as a lubricatingoil to be circulated within a screw compressor. JP 2004-323586 Adescribes that a zinc compound is added to a grease composition used inelectrical components of an automobile, and that at least one type ofzinc dialkyldithiophosphate, zinc dialkyldithiocarbamate, or zinc oxideis used as the zinc compound. JP H09-3473 A describes, phosphate esters,phosphite esters, zinc dialkyldithiophosphate, zincdialkyldithiocarbamate, and sulfur compounds as antiwear additives to beadded to the heat-resistant lubricating oil composition used in anautomobile engine. JP H08-143883 A describes a grease composition whichis suitable for use in a fan clutch bearing for an automobile, thegrease composition having high heat resistance and containing afluorosilicone oil as a base oil, and also describes zincdithiophosphate, zinc dithiocarbamate, phosphates, and alkyl sulfites asextreme-pressure additives that can be added. JP H05-230486 A describesa silicone grease composition containing zinc dialkyldithiophosphate,which is used in an overrunning clutch of an automobile starter.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: JP 2010-112235 A

Patent Document 2: JP 2004-323586 A

Patent Document 3: JP H09-3473 A

Patent Document 4: JP H08-143883 A

Patent Document 5: JP H05-230486 A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In response to the reduction in size and weight and increasingly harshusage conditions of clutches or torque limiter mechanisms of recentyears, and due to increases in the number of times that a starter for anautomobile engine is used, in particular, there is a demand for a greasecomposition which exhibits even higher friction characteristics and wearcharacteristics than conventional grease compositions. Therefore, anobject of the present invention is to provide a grease compositioncontaining a silicone oil as a base oil which exhibits high frictioncharacteristics and wear characteristics.

Means for Solving the Problems

As a result of conducting dedicated research, the present inventorsdiscovered that a grease composition obtained by adding a zinc salt (B)containing, at a mass ratio from 1:99 to 99:1, a zincdialkyldithiophosphate (B1) and a zinc dialkyldithiocarbamate (B2) asextreme-pressure additives to a silicone oil (A) has high frictioncharacteristics and wear characteristics and is suitable for use in apower transmission device, and in a starter overrunning clutch, inparticular.

Therefore, the present invention relates to a grease compositioncontaining: a silicone oil (A); and a zinc salt (B) containing, at amass ratio from 1:99 to 99:1, a zinc dialkyldithiophosphate (B1) and azinc dialkyldithiocarbamate (B2).

The content of the zinc salt (B) is preferably not less than 5 mass %and not greater than 30 mass % relative to the total amount of thecomposition. In addition, the zinc salt (B) preferably contains the zincdialkyldithiophosphate (B1) and the zinc dialkyldithiocarbamate (B2) ata mass ratio from 2:98 to 50:50.

The grease composition of the present invention may further contain oneor more types of thickeners (C) selected from lithium soaps and lithiumcomplex soaps.

The content of the silicone oil (A) is preferably in the range of notless than 50 mass % and not greater than 95 mass % relative to the totalamount of the composition, and the silicone oil (A) preferably containsan aromatic hydrocarbon group in the molecule.

In addition, the present invention relates to a grease composition for apower transmission device, the grease composition containing: a siliconeoil (A); and a zinc salt (B) containing, at a mass ratio from 1:99 to99:1, a zinc dialkyldithiophosphate (B1) and a zincdialkyldithiocarbamate (B2). Further, the grease composition of thepresent invention may optionally contain a partial ester (D) of apolyhydric alcohol to enhance the wear resistance.

Further, the present invention relates to a machine component in whichthe grease composition of the present invention is enclosed, and inparticular, a clutch or a torque limiter mechanism.

In addition, the present invention relates to an overrunning clutch inwhich the grease composition of the present invention is enclosed in aclutch cam chamber formed from an outer clutch and an inner clutch.

Effects of the Invention

The grease composition of the present invention exhibits a highcoefficient of friction and excellent enhancement of wear resistance.Further, the grease composition can be used in a wide temperature rangefrom low to high temperatures. In comparison to a case where zincdialkyldithiophosphate or zinc dialkyldithiocarbamate is used alone or acase where other sulfur-based additives are used, the grease compositionyields higher friction characteristics and wear characteristics. Byusing a specific combination of a zinc dialkyldithiophosphate and a zincdialkyldithiocarbamate, it is possible to satisfy the contradictoryrequirements of high friction and low wear required for an overrunningclutch, which was impossible with known products.

MODE FOR CARRYING OUT THE INVENTION

A first aspect of the present invention is a grease compositioncontaining: a silicone oil (A); and a zinc salt (B) containing, at amass ratio from 1:99 to 99:1, a zinc dialkyldithiophosphate (B1) and azinc dialkyldithiocarbamate (B2).

The silicone oil serving as component (A) of the present invention is aliquid at ambient temperature (25° C.) and may have a straight-chain ora branched-chain structure. The chemical structure thereof is notparticularly limited, but an example thereof is a polyorganosiloxane(silicone oil), which is a liquid at ambient temperature, represented bythe following general formula (I).

R¹ _(a)SiO_((4-a)/2)   (I)

(where the R¹ moieties are groups selected from monovalent hydrocarbongroups and may be the same as or different than one another. Here, a ispreferably from 1.8 to 2.2 and more preferably from 1.9 to 2.1 from theperspective of the ease of synthesizing liquid silicone.)

The R¹ moieties in Formula (1) above may each independently be analiphatic hydrocarbon group or an aromatic hydrocarbon group.Preferably, the silicone oil used in the grease composition of thepresent invention contains an aromatic hydrocarbon group as at least oneof the R¹ moieties.

The carbon number of the hydrocarbon group of R¹ may be from 1 to 8.Examples of R¹ include alkyl groups such as a methyl group, an ethylgroup, a propyl group, and a hexyl group, cyclohexyl groups such as acyclopentyl group and a cyclohexyl group, alkenyl groups such as a vinylgroup and an allyl group, aryl groups such as a phenyl group and a tolylgroup, aralkyl groups such as a 2-phenylethyl group and a2-methyl-2-phenylethyl group, and halogenated hydrocarbon groups such asa 3,3,3-trifluoropropyl group, a 2-(perfluorobutyl)ethyl group, a2-(perfluorooctyl)ethyl group, and a p-chlorophenyl group, and a methylgroup, a vinyl group, and a phenyl group are preferable. The siliconeoil particularly preferably contains a methyl group and a phenyl group.

Examples of specific silicone oils include the following silicones (1)to (3) due to their versatility as greases. The use of (1) methylphenylsilicone and (2) dimethylsilicone, which have a particularly highcoefficient of friction, is preferable.

Methylphenyl silicone:

Me₃SiO(Me₂SiO)_(l)(R²R³SiO)_(m)SiMe₃

(where Me is a methyl group; R² is a methyl group or a phenyl group; R³is a phenyl group; l is 0 or a positive integer; and m is a positiveinteger)

Specific examples include the following:

Me₃SiO(Me₂SiO)_(l)(MePhSiO)_(m)SiMe₃

and

Me₃SiO(Me₂SiO)_(l)(Ph₂SiO)_(m)SiMe₃

(2) Dimethylsilicone

Me₃SiO(Me₂SiO)_(n)SiMe₃

(where Me is a methyl group; and n is a positive integer)

(3) Fluorinated alkyl-modified silicone:

Me₃SiO(Me₂SiO)_(r)(MeR⁴SiO)_(s)SiMe₃

(where Me is a methyl group; R⁴ is a fluorinated alkyl group; r is 0 ora positive integer; and s is a positive integer)

A specific example is the following:

Me₃SiO(Me(CF₃CH₂CH₂)SiO)_(s)SiMe₃

Note that the above-described silicones (1) to (3) may be branchedsilicones in which a methyl group of a side chain is substituted withanother siloxane group, and a terminal group may be substituted with ahydroxyl group, a vinyl group, or the like.

A particularly preferable silicone oil is the methylphenyl siliconerepresented by (1) above. When the phenyl group content of thismethylphenyl silicone is less than 1 mol % relative to all organicgroups, the heat resistance becomes low, whereas when the contentexceeds 50 mol %, temperature-induced changes in viscosity become large,and the pour point also increases, while the torque at low temperaturesbecomes large. Therefore, the phenyl group content is preferably in therange from 1 to 50 mol %, more preferably from 3 to 25 mol %, andparticularly preferably from 3 to 10 mol %.

When the kinetic viscosity of the silicone oil used in the presentinvention is less than 20 cSt, the silicone oil is easily separated fromthe grease, whereas when the kinetic viscosity exceeds 10000 cSt, thetorque at low temperatures becomes large due to viscous resistance, andheat is generated at the time of high-speed revolution, which inducesthe degradation of the grease. Therefore, the kinetic viscosity ispreferably from 20 to 1000 cSt and more preferably from 50 to 500 cSt at25° C. The measurement of the kinetic viscosity is a value measured inaccordance with “JIS Z 8803”.

In the grease composition of the present invention, the content of thesilicone oil may be not less than 10 wt. % and not greater than 99 wt. %relative to the total amount of the composition, and the content ispreferably not less than 30 wt. % and not greater than 97 wt. % and morepreferably not less than 50 wt. % and not greater than 95 wt. %.

By using a silicone oil such as that described above as a base oil, agrease exhibiting the high friction required for an overrunning clutchcan be realized because the silicone oil is an oil agent with a highcoefficient of friction and exhibits excellent heat resistance andminimal temperature dependence of changes in viscosity.

The grease composition of the present invention contains, as anextreme-pressure additive, a zinc salt (B) containing, at a mass ratiofrom 1:99 to 99:1, a zinc dialkyldithiophosphate (B1) and a zincdialkyldithiocarbamate (B2). The combination of the component (B1) andthe component (B2) makes it possible to enhance the frictioncharacteristics and the wear characteristics of the silicone greasecomposition. Such high friction characteristics and wear characteristicsare not imparted when the component (B1) or the component (B2) is usedalone. In addition, such an effect is not imparted when another typicalsulfur-based additive is used.

The zinc dialkyldithiophosphate of the component (B1) used in thepresent invention is preferably represented by the following generalformula (II).

[R⁵O)₂P(═S)—S]₂—Zn   (II)

(where R⁵ is a primary or secondary alkyl having from 1 to 24 carbons.R⁵ is particularly preferably a primary or secondary alkyl group havingfrom 3 to 12 carbons.)

The zinc dialkyldithiocarbamate of the component (B2) used in thepresent invention is preferably represented by the following generalformula (III).

[R⁶ ₂N—C)(═S)—S]₂—Zn   (III)

(where R⁶ is a primary or secondary alkyl group having from 1 to 24carbons. R⁶ is particularly preferably a primary or secondary alkylgroup having from 1 to 8 carbons.) In particular, R⁶ is preferably amethyl group, an ethyl group, and a propynyl group)

In the grease composition of the present invention, the content of thezinc salt (B) may be not less than 1 mass % and not greater than 50 mass% relative to the entire composition, and the content is preferably notless than 5 mass % and not greater than 40 mass % and more preferablynot less than 5 mass % and not greater than 30 mass %.

The mass ratio of the zinc dialkyldithiophosphate and the zincdialkyldithiocarbamate is preferably from 1:99 to 40:60 and morepreferably from 2:98 to 50:50. When the proportion of the zincdialkyldithiophosphate is greater than that of the zincdialkyldithiocarbamate, the coefficient of friction decreases.Therefore, the proportion of the zinc dialkyldithiophosphate ispreferably smaller than the proportion of the zincdialkyldithiocarbamate.

The grease composition of the present invention may further contain anorganic or inorganic thickener. The grease composition preferablycontains a lithium soap and/or a lithium complex soap, which are organicthickeners, as a component (C). By containing a lithium soap, the greasecomposition can be imparted with heat resistance, shear stability, andwater resistance. In addition, by containing a lithium complex soap, thegrease composition can be imparted with even greater shear stability andheat resistance than those imparted by a lithium soap.

The grease composition of the present invention may further contain anorganic thickener such as a urea compound or an inorganic thickener suchas silica and bentonite.

In the grease composition of the present invention, the content of thethickener may be not less than 1 mass % and not greater than 30 mass %relative to the total amount of the composition, and the content ispreferably not less than 2 mass % and not greater than 25 mass % andmore preferably not less than 2.5 mass % and not greater than 20 mass %.

The grease composition of the present invention may optionally furthercontain a partial ester of a polyhydric alcohol as a component (D). Thisester is used as an oily agent and adsorbs to one of the metals in ahydroxyl group portion. This may prevent the hydrocarbon groupsextending from the ester bonds from coming into contact with othermetals and thereby enhance the lubricating action. Note that, asnecessary, the content of the component (D) may be not greater than 30mass %, or the component (D) may not be contained at all.

The partial ester of the polyhydric alcohol described above can beobtained through a condensation reaction of a polyhydric alcohol and amonovalent fatty acid. Specific examples of polyhydric alcohols includeglycerin, trimethylol propane, diglycerin, erythritol, pentaerythritol,triglycerin, sorbitol, and mannitol. Pentaerythritol is particularlypreferable. In addition, the monovalent fatty acid forming a partialester with the polyhydric alcohol preferably has from 5 to 20 carbons,and caproic acid (C6), enantoic acid (C7), and caprylic acid (C8) areparticularly preferable examples. The partial ester of the polyhydricalcohol may have a pour point lower than 40° C. Specifically, thepartial ester may be an ester of a pentaerythritol fatty acid (C6-C8).

In the grease composition of the present invention, the content of thepartial ester of the polyhydric alcohol may be not less than 0 mass %and not greater than 30 mass % relative to the total amount of thecomposition, and is preferably not less than 0 mass % and not greaterthan 20 mass %. The content of the component (D) may be 0 mass % asnecessary.

The grease composition of the present invention may further containadditives which are ordinarily used in grease compositions, such asantioxidants, corrosion inhibitors, metal deactivators, detergentdispersant extreme-pressure additives other than those described above,anti-forming agents, anti-emulsifiers, and oiliness enhancers. These maybe used alone or as a mixture of two or more types thereof. The contentthereof may be from 0 wt. % to 5 wt. % relative to the total amount ofthe composition as long as the effect of the present invention is notinhibited.

The grease composition of the present invention is used in thelubricating part of a power transmission device. A power transmissiondevice refers to a series of mechanisms for transmitting the power ofthe engine to the wheels. A power transmission device includes a clutch,a transmission, a propeller shaft, a coupling, a final reduction gear, adriving axle, and the like. The grease composition of the presentinvention is preferably used in the lubricating part of a clutch, andmore preferably in the lubricating part of a starter overrunning clutch.

A second aspect of the present invention is a machine part in which thegrease composition of the present invention is enclosed, and preferablya clutch or a torque limiter mechanism. A starter overrunning clutch isparticularly preferable in which the grease composition of the presentinvention is enclosed. A starter overrunning clutch includes acylindrical outer clutch and a cylindrical inner clutch disposedconcentrically on the inner peripheral side of the outer clutch, and aclutch roller is displaced in the circumferential direction within aclutch cam chamber formed by the outer clutch and the inner clutch. Thegrease composition of the present invention can be enclosed in thisclutch cam chamber.

Another aspect of the present invention is a method of increasing thefriction of the lubricating part of a power transmission device and/orreducing the wear of the lubricating part, wherein the greasecomposition of the present invention is enclosed in the powertransmission device. An additional example is a method of increasing thefriction of the lubricating part of an overrunning clutch and/orreducing the wear of the lubricating part, wherein the greasecomposition of the present invention is enclosed in a clutch cam chamberformed by the outer clutch and the inner clutch of the overrunningclutch.

In addition, another aspect of the present invention is the use of thegrease composition of the present invention for increasing the frictionof the lubricating part of a power transmission device and/or reducingthe wear of the lubricating part. An additional example is the use ofthe grease composition of the present invention for increasing thefriction of the lubricating part of an overrunning clutch and/orreducing the wear of the lubricating part.

INDUSTRIAL APPLICABILITY

The grease composition of the present invention has a high coefficientof friction and therefore has a high torque transmission as well as highwear resistance. The grease composition is therefore suited for use in alubricating part of a clutch or a torque limiter mechanism which is usedwith high frequency, and a starter overrunning clutch, in particular.

EXAMPLES

The present invention will be described specifically using examples andcomparative examples hereinafter, but the present invention is notlimited to the examples below.

Preparation of Grease Composition

A methylphenyl silicone (A), a zinc dialkyldithiophosphate (B1) and azinc dialkyldithiocarbamate (B2), and a thickener (C) (“Li soap” or “Licomplex soap”) were mixed in the composition listed in Table 2 (mixed toadjust the consistency to approximately 300). After the entire mixturewas stirred, the mixture was adjusted to grades of consistency Nos. 1and 2 using three roll mills to prepare the grease compositions ofExamples 1 to 7 and Comparative Examples 1 to 7.

Note that the details of each component are as follows.

Methylphenyl silicone (A): available from Dow-Corning Corporation, DC510-100 mm²/s (25 ° C.)

Zinc dialkyldithiophosphate (B1): bis(2-ethylhexyl)zinc dithiophosphate

Zinc dialkyldithiocarbamate (B2): zinc dialkyldithiocarbamate

Thickener (C):

Li soap: lithium stearate

Li complex soap: 4:1 mixture of lithium(12-hydroxystearate) anddilithium azelate salt

Friction Test and Wear Test

The coefficient of friction was measured in accordance with a barbellplate test, and the size of wear marks was further measured.Specifically, an opposite material with a barbell shape was brought intocontact with the plate surface, and the plate was rotated while applyinga load in this state so as to slide the surface of the opposite materialand the plate surface with continuous contact therebetween. The oppositematerial was a Fe—Cr alloy steel formed from two discs 13 mm in diameterand 2 mm thick and a shaft connecting the central portions of these twodiscs. The portion of the opposite material which was in sliding contactwith the plate surface was the peripheral edge portion of the discs.

(1) Test Piece Material

Barbell-type test piece: SCr415

Plate-type test piece: SUJ2

(2) Test Method

A state of cylindrical line contact was assumed so as to achieve astable state of contact with little one-sided contact even underrelatively low surface pressure conditions. The test conditions of thefriction test and the wear test are shown in Table 1. The coefficient offriction was measured by measuring the torque in the sliding directionwith a load cell. The wear width (μm) of wear marks was measured withthe measurement function of a stereoscopic microscope (available fromOlympus, model DP-21).

(3) Test Conditions

TABLE 1 Item Conditions Test conditions of friction test Load (N) 800Surface pressure (MPa) 1015 Sliding rate (m/s) 0.388 Testing time (s) 15Test temperature (° C.) −30 Test conditions of wear test Load (N) 40Surface pressure (MPa) 300 Sliding rate (m/s) 3.50 Testing time (s) 150Test temperature (° C.) 25

(4) Assessment of Effects

Friction Test

As a result of measuring the coefficient of friction at −30° C. withthis barbell plate test, cases with a coefficient of friction of 0.160or greater were assessed as “good”, and cases with a coefficient offriction of less than 0.160 were assessed as “poor”.

Wear Test

According to this barbell plate test, cases in which the wear mark wassmaller than 1000 μm were assessed as “good”, and cases in which thewear mark was 1000 μm or greater were assessed as “poor”.

The results of using the grease compositions of Examples 1 to 7 andComparative Examples 1 to 7 in friction tests and wear tests are shownin Table 2.

TABLE 2 Examples 1 2 3 4 5 6 7 Base oil: methylphenyl silicone (A) (mass%) 73.7 73.7 73.3 72.8 72.4 71.4 73.3 Zinc dialkyldithiophosphate (B1)(mass %) 2.0 4.0 2.5 3.0 3.5 1.0 2.5 Zinc dialkyldithiocarbamate (B2)(mass %) 11.3 9.3 11.3 11.3 11.3 15.0 11.3 Li soap (C) (mass %) 13.013.0 12.9 12.9 12.8 12.6 0.0 Li complex soap (C) (mass %) 0 0 0 0 0 0 0Coefficient of friction 0.182 0.170 0.171 0.165 0.165 0.165 0.166 0.175∘ ∘ ∘ ∘ ∘ ∘ ∘ Wear marks (μm) 930 630 810 880 630 960 790 Assessment ofwear test results ∘ ∘ ∘ ∘ ∘ ∘ ∘ Comparative Examples 1 2 3 4 5 6 7 Baseoil: methylphenyl silicone (A) (mass %) 83.3 81.6 78.2 80.75 77.137572.25 68.0 Zinc dialkyldithiophosphate (B1) (mass %) 2.0 4.0 8.0 0.0 0.00.0 0.0 Zinc dialkyldithiocarbamate (B2) (mass %) 0.0 0.0 0.0 5.0 9.315.0 20.0 Li soap (C) (mass %) 14.7 14.4 13.8 14.25 13.6125 12.75 12.0Coefficient of friction 0.176 0.166 0.125 0.199 0.195 0.180 0.170Assessment of friction test results ∘ ∘ x ∘ ∘ ∘ ∘ Wear marks (μm) 16801230 820 1570 1300 1230 1120 Assessment of wear test results x x ∘ x x xx

As described above, high friction characteristics and wearcharacteristics were achieved only when a combination of a zincdialkyldithiophosphate and a zinc dialkyldithiocarbamate was used as anextreme-pressure additive. On the other hand, either the frictioncharacteristics or the wear characteristics were poor when a zincdialkyldithiophosphate or a zinc dialkyldithiocarbamate was used alone.

1. A grease composition comprising: a silicone oil (A); and a zinc salt(B) containing, at a mass ratio from 1:99 to 99:1, a zincdialkyldithiophosphate (B1) and a zinc dialkyldithiocarbamate (B2). 2.The grease composition according to claim 1, wherein a content of thezinc salt (B) is not less than 5 mass % and not greater than 30 mass %relative to the entire grease composition.
 3. The grease compositionaccording to claim 1, wherein the zinc salt (B) contains the zincdialkyldithiophosphate (B1) and the zinc dialkyldithiocarbamate (B2) ata mass ratio from 2:98 to 50:50.
 4. The grease composition according toclaim 1, further comprising one or more types of thickeners (C) selectedfrom the group consisting of lithium soaps and lithium complex soaps. 5.The grease composition according to claim 1, wherein a content of thesilicone oil (A) is in a range of not less than 50 mass % and notgreater than 95 mass % relative to the total amount of the greasecomposition, and the silicone oil (A) has an aromatic hydrocarbon groupin a molecule.
 6. The grease composition according to claim 1, furthercomprising a partial ester (D) of a polyhydric alcohol.
 7. The greasecomposition according to claim 1, wherein the grease composition is agrease composition for a power transmission device.
 8. A machinecomponent in which the grease composition according to claim 1 isenclosed.
 9. The machine component according to claim 8, wherein themachine component is a clutch or a torque limiter mechanism.
 10. Anoverrunning clutch, wherein the grease composition according to claim 1is enclosed in a clutch cam chamber formed from an outer clutch and aninner clutch.
 11. The grease composition according to claim 2, whereinthe zinc salt (B) contains the zinc dialkyldithiophosphate (B1) and thezinc dialkyldithiocarbamate (B2) at a mass ratio from 2:98 to 50:50. 12.The grease composition according to claim 11, further comprising one ormore types of thickeners (C) selected from the group consisting oflithium soaps and lithium complex soaps.
 13. The grease compositionaccording to claim 12, wherein a content of the silicone oil (A) is in arange of not less than 50 mass % and not greater than 95 mass % relativeto the total amount of the grease composition, and the silicone oil (A)has an aromatic hydrocarbon group in a molecule.
 14. The greasecomposition according to claim 13, further comprising a partial ester(D) of a polyhydric alcohol.